Before discussing the effects of opioid drugs on the baby, it is important to exclude events that are not caused by opioids:

1. they do not cause congenital abnormalities or defects;
2. they do not interfere with uterine contractions or the progress of labour;
3. they have no harmful effects on the baby's circulation during labour (although they can reduce the normal variability in heart rate - the significance of which is unknown).

As stated earlier in this chapter, all opioid drugs readily cross the placenta to enter the baby's circulation. For many opioids, the blood concentration in the baby is approximately 70 per cent of that which occurs in the mother. Once in the bloodstream of the fetus, opioid drugs are then free to pass to the various organs before being eliminated by the liver and kidneys - in just the same way as they do in the adult. Naturally, a proportion of the opioid drug is also free to interact with the baby's opioid receptors. What effects, then, are opioids likely to have, and are they important? Once again it is worthwhile recalling activities which are mediated via the opioid receptors.

It is very likely (though unproven) that opioids do reduce the response to pain in the newborn (or fetus, for that matter). We cannot, of course, know what pain a baby normally experiences at birth - so we cannot tell whether analgesia is either beneficial or harmful. There is at least one instance, however, where opioid analgesia has been shown to be useful: in the very premature infant requiring ventilatory assistance. In this case, it is necessary to place a tube into the trachea (windpipe) so that the lungs can be inflated by a ventilator. This is a stimulating procedure and causes the blood pressure to rise: in some cases, it is thought to provoke intracranial bleeding. Opioid drugs reduce this blood pressure response to intubation and many neonatologists nowadays prefer to perform this procedure in babies who are well sedated.

As discussed already, opioid drugs do depress breathing and they certainly have the potential to produce respiratory depression in the newborn. Some writers, however, have grossly exaggerated the importance of this side effect by stating that drugs must be to blame if the baby is 'flat' at birth, or does not cry straight away. Such statements are a gross distortion of the truth.

The fact is that opioid drugs - especially when given in the kinds of doses used today - are very rarely responsible for causing serious respiratory depression. A more common, and potentially more serious, cause for a delayed onset in breathing at birth is asphyxia (a reduction in fresh blood supply to the baby prior to delivery due to such causes as cord compression, or the cord being wound around the neck). Asphyxia can occur at any time during labour - irrespective of whether the mother has received any drugs. Nowadays, asphyxia is the most common indication for obstetric intervention (e.g. caesarean section or an instrumental delivery). An asphyxiated baby needs to be delivered, resuscitated and given oxygen. Any depression in breathing noticed at birth in such cases is secondary to lack of oxygen prior to delivery - not to drugs.

As explained under side effects, respiratory depression is easily reversed. If there is any doubt about whether a baby is 'under the influence' of an opioid drug, or not, then a simple injection of an opioid antagonist (e.g. naloxone) will quickly settle the matter. If opioids are to blame, the baby will wake up: if not, it won't! Naloxone can be given into the umbilical cord or a muscle at any time after delivery. It is not only false, but irresponsible, to promote the view that opioid drugs are in some way a threat to survival. If this was the case, they would have been condemned and discarded long ago. It should also be remembered that just because a drug (or any other chemical substance, for that matter) can be detected in the body, it does not necessarily mean that it will have a deleterious effect. After all, if you happen to have had a drink of coffee or sherry before going into labour, then caffeine (or alcohol) can be detected in your baby for many hours after delivery,

Readers can be reassured, therefore, about the overall safety of opioid drugs in childbirth. There remains some question as to whether opioids can cause more subtle, long lasting effects on the intellectual and behavioural development of the newborn. This is a difficult and controversial area, and one in which the author is not an authority, however, some of the objections that have been made seem both nebulous and dubious.

First, opioid drugs have been said to interfere with infant bonding and breastfeeding in the first few days of life. If this were so, one would expect weight gain in the first few days of life to be less among those infants whose mothers had received opioid drugs during labour. But this has not proved to be the case. Unfortunately, most studies have not taken into account all of the variables which can influence early feeding patterns. Furthermore, milk production is poor in the first 48 hours, and infants invariably lose weight to begin with. So, even if there is any opioid effect on suckling behaviour in the first day or so this is not likely to be of any real importance.

Second, opioid drugs may have some transient effects on the neurobehavioural development of the newborn. Once again, the relevance of these findings seems equivocal. Several different tests have been introduced to try and measure the effects of drugs given to the mother during labour. These are too complicated to discuss here in detail, but include assessments of muscle tone, state of arousal, the ability to suppress unimportant stimuli (e.g. ringing a bell) and the ability to respond to useful stimuli (e.g. suckling). Unfortunately, these tests are not specific and are influenced by many other factors quite apart from drugs.

Other flaws in many of the studies which have attempted to demonstrate an adverse influence of drugs on the newborn can be summarised as follows:

(a) sample sizes have been too small to generalise about the population as a whole;

(b) several different drugs have been given (including sedatives);

(c) drug concentrations have not been measured at birth;

(d) there has been no control group (a group which has received no medication);

(e) measures have not always been taken to exclude observer bias (the investigator has known beforehand what drugs have been given).

One of the most detailed and reliable series of studies concerning the influence of pethidine on the newborn has come from St Mary's Hospital Medical School, London. In these studies, a group of infants whose mothers had received pethidine during labour was compared with a control group whose mothers had received no pain relief at all. It was found that there was no difference in neurobehaviour between the two groups (measured at four time periods) during the first six weeks of life. In other words, pethidine had no detectable effect overall on neurological or behavioural development. There was, however, some depression in function with the higher doses of pethidine. Since there was no overall difference between the two groups, this means that infants who were exposed to low doses of pethidine must have performed better than those who were not exposed to any drugs at all. This raises the possibility that any minor adverse effect of pethidine on the newborn was more than offset by a positive effect on the interaction between the mother and her baby. By helping to reduce emotional stress in the mother, pethidine may actually have benefited the baby.

At least two other major studies have found no long-term effects of pain relief (of any kind) on the behaviour, neurological development or scholastic achievement of children up to the age of five years. There are no scientific grounds to believe, therefore, that analgesic drugs administered during labour have an adverse influence on the ultimate well being of the child.